Pirs (Russian: Пирс, meaning "pier") -- also called "Stykovochny Otsek 1" or "SO-1" (Russian: Стыковочный отсек, "docking module", or DC-1 (docking compartment) -- is one of the two Russian docking compartments originally planned for the ISS. Pirs was launched in August 2001. It provides the ISS with one docking port for Soyuz and Progress spacecraft, and allows egress and ingress for spacewalks by cosmonauts using Russian Orlan space suits.

A second docking compartment, "Stykovochniy Otsek 2" or SO-2, was initially planned with the same design. However, when the Russian segment of the ISS was redesigned in 2001, the new design no longer included the SO-2, and its construction was canceled.[1] After another change of plans the SO-2 module finally evolved into the Poisk module, which was added to the ISS in 2009.

The Docking Compartment has two primary functions: Provide a docking port and serve as an airlock.

The docking port can accommodate one Soyuz-TMA or one Progress-M spacecraft. Visiting spacecraft can deliver people and cargo to and from to the space station. In addition, the Docking Compartment can transport fuel from the fuel tanks of a docked Progress resupply vehicle to either the Zvezda Service Module Integrated Propulsion System or the Zarya Functional Cargo Block. It can also transfer propellant from Zvezda and Zarya to the propulsion system of docked vehicles—Soyuz and Progress.

The Pirs docking compartment was manufactured by RKK Energia. The Docking Compartment is similar to the Mir Docking Module used on the Mir space station. The docking compartment's planned lifetime as part of the station was five years.

The 3,580-kilogram Pirs Docking Compartment is attached to the nadir (bottom, Earth-facing) port of the Zvezda service module. It docked to the International Space Station on September 16, 2001, and was configured during three spacewalks by the Expedition 3 crew. Two Strela cargo cranes were later added by the STS-96 and STS-101 missions, carried up on Integrated Cargo Carriers and installed during EVAs.

Pirs was originally scheduled to be detached from the nadir (bottom) port of the Zvezda module in 2014[2] to make room for the Russian Multipurpose Laboratory ModuleNauka.[3] This has been delayed to 2017 due to a series of launch failures with the Proton rocket leading to delays in the launch of Nauka.[4] When that happens, Pirs could become the first permanent ISS module to be decommissioned, and would be destroyed during atmospheric re-entry.